Researches are actively carried out on the development of a rechargeable secondary battery in high-tech fields such as a digital camera, a cellular phone, a notebook computer, a hybrid vehicle, etc. The secondary battery includes a nickel-cadmium battery, a nickel-metal hydride battery, a nickel-hydrogen battery and a lithium secondary battery. Among them, the lithium secondary battery is used as a power source of portable electronic devices or used in high-power hybrid vehicles by being connected in series. The lithium secondary battery is widely used because its operation voltage is 3 times higher than the nickel-cadmium battery or the nickel-metal hydride battery and its energy density per unit weight is also superior.
The lithium secondary battery can be fabricated into various shapes, including cylindrical and prismatic shapes. Recently, the flexible pouch-type secondary battery is drawing a lot of attentions.
FIG. 1 shows a general pouch-type secondary battery 10 and a sealing apparatus 20 for sealing the pouch-type secondary battery 10.
Referring to FIG. 1, the pouch-type secondary battery 10 includes a pouch case consisting of an upper pouch 15a and a lower pouch 15b and a lead 17. The lead 17 extends from an electrode assembly (not shown) installed inside the pouch 15a, 15b so as to protrude outside the pouch 15a, 15b. And, a sealing portion 15 is formed along the edge of the pouch 15a, 15b so as to hermetically seal the pouch 15a, 15b. 
The sealing apparatus 20 is installed to be ascendable and descendible by an ascending/descending means 30 provided above and below the pouch sealing portion 15. The sealing apparatus 20 compresses the sealing portion 15 as it ascends and descends. At this time, the sealing apparatus 20 heats the sealing portion 15 with heat generated by a heating means (not shown) equipped therein. That is to say, as the sealing apparatus 20 heats and compresses the sealing portion 15, the polymer of the pouch 15a, 15b is melted and sealing is achieved.
In order to prevent the leakage of an electrolyte inside the pouch 15a, 15b from leaking through the joint of the lead 17 and the pouch 15a, 15b, a film 18 of a thin resin layer is coated on the lead 17 as shown in FIG. 2. In FIG. 2, (a) is a view of the lead seen from above and (b) is a cross-sectional view along the line B-B′ of (a). However, despite the coating of the film 18, the lead 17, which has a thickness of 50-1000 μm in general, is not completely sealed by the heat sealing, because the upper and lower pouches 15a, 15b are disposed as shown in FIG. 3, at the portion where the lead 17 corresponds to the upper and lower pouches 15a, 15b and the electrolyte may leak. That is to say, because the lead 17 has a predetermined thickness as descried above, the joined portion gets loose due to the thickness of the lead 17 after the joining of the pouch 15a, 15b. If the upper and lower cases around the lead 17 are joined incompletely as described above, the electrolyte may leak around the lead, thereby degrading electrical properties of the secondary battery after repeated charging and discharging. In order to solve this problem, an improved sealing apparatus which seals the lead 17 after disposing it between upper and lower heating blocks is disclosed, as shown in FIG. 4.
Referring to FIG. 4, the sealing apparatus 40 is equipped with upper and lower heating blocks 41, 42 equipped with a heating means (not shown) and sealing grooves 41a, 42a on which a lead 17 is seated during sealing are formed on the portions of the heating blocks 41, 42 corresponding to the lead 17 disposed between upper and lower cases 15a, 15b. The sealing grooves 41a, 42a are configured such that the lead 17 is seated thereon and the portions other than the lead 17 are sufficiently compressed to ensure sealing. The lead 17 is sealed with its thickness compensated by the sealing grooves 41a, 42a. 
However, because the lead 17 is not constantly positioned on the sealing grooves 41a, 42a in the actual sealing process, the sealing grooves 41a, 42a usually have a margin d considering the tolerance of the lead 17, as shown in FIG. 5. In general, the sealing grooves 41a, 42a are formed to have a width about 10% wider than the width of the lead 17.
As indicated by the dashed arrows in FIG. 6, the sealing process can be performed even when the position on which the lead 17 is seated is not constant. However, due to the margin, an unsealed portion not compressed by the sealing apparatus 40 is formed by the difference in the widths of the sealing grooves 41a, 42a and the lead 17, i.e., the margin. As the thickness of the lead 17 is larger and as the depth and width of the sealing grooves 41a, 42a increase, the unsealed portion becomes larger too. In addition, if the sealing condition or the lead condition changes, the void space, or gap, between the sealing groove 42a and a film 18 may not be filled as shown in FIG. 6. In this case, the sealing is not achieved completely and electrolyte leakage may occur.